Orthogonal frequency division multiplex (OFDM) is a multi-carrier signal generation method whereby data are sent simultaneously over N equally spaced carrier frequencies using Fourier transform techniques for modulation and demodulation. By proper choice of frequencies, OFDM can squeeze multiple modulated carriers into a prescribed band while preserving orthogonality to eliminate inter-carrier interference (ICI). When compared to single-carrier modulation, OFDM has some distinct advantages.
Unfortunately, in the OFDM signal, the constructive summation of N sinusoidal carriers can result in peak envelope power that is as much as N times the mean envelope power for unencoded signals. A large peak-to-average power ratio (PAPR) necessitates the use of linear amplifiers, which are inefficient. Furthermore, the amplifiers must typically be run at some back-off from compression in order to reduce the distortion introduced by clipping, further reducing efficiency. Amplifier back-off attenuates the transmitted signals, thereby placing greater demands on receiver sensitivity.
Thus, what is needed is a method and apparatus for reducing PAPR in an OFDM signal. Preferably, the method and apparatus will reduce the PAPR substantially, while minimizing the computational complexity required to achieve the reduction.